In using ultrasound for imaging, therapy or other medical uses, ultrasound energy or waves are transmitted through a medium and can reflect, scatter or otherwise attenuate when they reach a surface or border having a significant difference in acoustic impedance. For example, in ultrasound imaging of the human body, ultrasound waves may be applied externally (e.g. by placing a transducer on the skin) or internally (e.g. by placing a transducer within a vessel or organ), and travel through the body's internal fluids, which is a large proportion of water. When the waves strike a bone, organ or other body portion that provides an acoustic interface—i.e., a border of two significantly different acoustic impedances—then the waves are reflected or otherwise attenuated. A transducer (which may be the same transducer that supplied the ultrasound waves or another) receives the reflected or attenuated waves, and an image of a portion of the body can be generated.
An external transducer is placed against the skin with a gel useful as a coupling medium so that little or no reflection or other attenuation from the boundary between the air over the patient and the skin occurs. When a transducer is placed within the body, commonly it is inside a protective envelope, such as a tube, catheter or similar housing or enclosure. The material of such an envelope may be selected for its similarity in acoustic impedance to that of bodily fluids, so that there is little or no attenuation as ultrasound waves travel from that material to the fluids or tissues of the body. The inner pocket or volume of the body within which the transducer is placed needs a coupling medium having an acoustic impedance similar to that of the envelope material and the body's fluids, to allow maximum transmission. Without such a medium, e.g. if the inside of the body simply includes air or another gas, significant reflection or other attenuation will occur when the ultrasound energy from the transducer hits the boundary where the gas meets the material of the envelope. Suitable coupling media include biocompatible fluids such as saline, oils such as mineral oil or castor oil, alcohols, and other fluids.
Of course, some coupling media (e.g. saline) can be corrosive over time, particularly of metallic materials. If a corrosive coupling medium is to be used, a drawback is that degradation of part(s) of the transducer or other aspects of the device (e.g. structure used to turn or move the transducer) may occur. An unacceptably short shelf life for the product may thus result with such media. Thus, the relatively low-cost medium of saline has significant downside to a practical internal transducer product.
Embodiments of internal transducer products using piezoelectric motors to turn or otherwise move a transducer have also been suggested. Applicable piezoelectric motors generally need dry conditions to operate, as they require a high friction contact area between a stator and a clutch. If fluid touches that contact area or interface, the friction will be substantially reduced, thereby also reducing the torque output of the motor Accordingly, in such embodiments to prevent fluid from contacting the motor a seal is included between the motor and the transducer, to prevent leakage of the coupling medium from the volume around the transducer toward the motor. Such seals can fail over time, which is one potential factor in shelf life for such products.